The Long Term Evolution (LTE) scheme is a succeeding communication scheme of the Wideband Code Division Multiplexing Access (WCDMA) scheme, the High-Speed Downlink Packet Access (HSDPA) scheme, the High-Speed Uplink Packet Access (HSUPA) scheme, and the like. The 3GPP that is a WCDMA standardization organization has developed the LTE scheme and created the specifications of the LTE scheme.
In addition, the 3GPP is currently developing the LTE-Advanced scheme as a succeeding communication scheme of the LTE scheme. The requirements of the LTE-Advanced scheme are described in Non-Patent Document 1.
In the LTE-Advanced scheme, the implementation of “Carrier aggregation (CA)” communications has been agreed as one of the requirements.
If the “CA communications” are implemented, mobile stations each can receive downlink signals by using multiple carriers at the same time, and can transmit uplink signals by using multiple carriers at the same time. Each carrier used in the operation of the carrier aggregation is referred to as a Component Carrier (CC).
In a mobile communication system including multiple cells, a mobile station UE (User Equipment) is configured to continue communications by switching cells when moving from one cell to another cell. Such cell switching is referred to as “handover.”
In general, a mobile communication system is configured that a mobile station performs handover to a neighboring cell when the radio quality of a signal from the neighboring cell becomes higher than the radio quality of a signal from a serving cell.
In this respect, for example, the received power of a signal is used as the above radio quality of the signal. More specifically, the received power of the signal is the received power of a downlink reference signal (RSRP: Reference Signal Received Power) sent from the neighboring cell or serving cell, for example (see Non-Patent Document 2 TS36.214, V8.7.0 for the definition of RSRP).
Instead of the RSRP, the radio quality of a downlink reference signal (RSRQ: Reference Signal Received Quality), the SIR (RS-SIR) of a downlink reference signal, CQI (Channel Quality Indicator), CSI (Channel State Information), or the like is used as the aforementioned radio quality of the signal in some cases.
With reference to FIG. 7 and FIG. 8, specific explanation is provided for one example of such handover procedure. In the following explanation, the received power of the signal (RSRP) is used as the radio quality of the signal.
As shown in FIG. 7, a mobile station UE measures the received powers of signals from a serving cell and a neighboring cell in step S1. In addition, in parallel with the measurement, the mobile station UE may perform cell search to detect an undetected neighboring cell. The cell search and the measurement of the ratio qualities (received powers) of the serving cell and the neighboring cell in this process may be collectively referred to as Measurement.
In step S2, the mobile station UE judges whether or not the received power of the signal from the neighboring cell satisfies the following (Equation 1):Received Power of Signal from Neighboring cell+Hysteresis>Received Power of Signal from Serving cell  (Equation 1).
When judging that the above (Equation 1) is satisfied, the mobile station UE notifies a network of “Measurement Report (Event A3)” for reporting the result of the above measurement.
Specifically, as shown in FIG. 8, the mobile station UE measures the received powers of signals from the serving cell (cell A) and a neighboring cell (cell B) which is a measurement object, and judges whether or not to send a notification of the above measurement result, by using “hysteresis [dB]” and “TTT (Time To Trigger) [ms]” which are informed in advance.
Specifically, in FIG. 8, the mobile station UE judges that the notification of the above measurement result (Measurement report) should be sent, if the received power (received quality) of the signal from the cell B is kept higher than the received power (received quality) of the signal from the cell A, by “hysteresis” or more, continuously for the predetermined time period “TTT” or longer.
Here, the “hysteresis” is a value set to avoid frequent handover from the serving cell to a neighboring cell at the cell boundary, and may take either of a positive value and a negative value. In general, a negative value is set as the “hysteresis.”
Then, upon receipt of the notification of the event A3, the network determines that the mobile station UE should perform handover to the cell related to the received “Measurement Report (Event A3)” in step S3.
Here, (Equation 1) may take another form such as the following (Equation 2). In the case of (Equation 2), both the hysteresis and offset work as hysteresis:Received Power of Signal from Neighboring Cell−Hysteresis>Received Power of Signal from Serving Cell+Offset  (Equation 2).
In the general operation of the CA communications, the mobile station UE performs for each CC the foregoing steps of measuring the received powers of signals from the serving cell and the neighboring cell, and sending Measurement report.